0. Japanese isn't easy.
According to the Foreign Service Institute's language ranking, Japanese is the hardest language to learn for English speakers; you would find an easier time learning Arabic or Cantonese. In my class, only two (including myself) really learned the language; everyone else just barely scraped by.
1. Your Japanese will always be sub-par compared to the natives.
Note that my Japanese skill is of a very high level - I was fluent after just 1 year of study, and I passed the JPLT level 1 on my first try. Nevertheless, Japanese is still not my native language, and although I am fluent - I don't sound as intelligent as I do in English. Due to this mis-perception, I am consistently evaluated worse by Japanese natives compared to English natives (note that the former are more numerous than the latter). The gist is that you will not find success easy in Japan. A better way to go is to be successful in America first, and then get seconded instead - you can learn Japanese later, but you don't even have to.
2. Knowing Japanese will not give you an advantage.
Whether you are bilingual or not will not help you get promoted. Knowing Japanese will only help you to understand icky politics in your workplace that you probably would rather not understand. Things you don't want to hear will float into your head, while things you should hear will require straining to comprehend. All that time you spend learning this language will essentially be for naught.
3. You will face opposition in the form of subtle discrimination.
Look out for the guys who can't speak English at all - they will have a bone to pick. It's not fun, this kind of situation.
Instead of majoring in Japanese (or Chinese), I would recommend one of the hard sciences (you won't get a chance to learn those ever again). You can just take language classes on the side.
